12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.

Study on the Fluvial Geomorphological Meanings of Valley Plain in the River and Estimation of theExtreme River Discharge
Masaki Wakui, Shoji Fukuoka
Research and Development Initiative, Chuo University
Tokyo, Japan

ABSTRACT

and roughness coefficient at the time of sediment deposition must be
estimated, the precision of flow rate calculation models is low.

In every region of the world, the number of large-scale natural disasters
is increasing year by year. According to a World Resources Institute(WRI) report, in 15 years, the economic impact of such disasters will beapproximately five times what it is now.
In 2007, the European Union (EU) issued Directive 2007/60/EC on theassessment and management of flood risks. In the directive, theprobabilities of high-frequency to low-frequency or extreme floods are
expressed in terms of annual exceedance probabilities (1/2 to 1/10,000).
Risk management is being implemented for floods of varying
magnitude. Recently in Japan, efforts have begun to officially announce
areas at risk of flooding under the assumption of an extreme rainfall
event with an annual exceedance probability of 1/1000 as the maximum
probable external force.
Current practice around the world is to use annual exceedance
probability to assign the magnitude of low-frequency floods. In thisstudy, however, we attempt to estimate the maximum flood discharge(extreme discharge) of actual floods that have occurred within the
recent epoch (that is, within approximately the last 10,000 years) basedon geomorphological and geological traces of rivers that flow througherosional valley-floor (valley plain). We believe this to be analternative method for estimating maximum probable external force.

KEY WORDS: valley plain , terrass, Yosasagawa Riv., extremal
discharge,

INTRODUCTION
The goal of this study was to estimate the discharge of largescale
historical floods on the basis of geomorphological and geological traces.
Fluvial geomorphology is formed through the sediment deposition anderosive action of water flowing through river channels. Efforts toreconstruct ancient flooding events on the basis of traces of deposition
action began in the field of paleohydrology in the mid-twentiethcentury; successful reconstruction of flood history has been achieved
through using the stratigraphy and radiocarbon dating of slackwater
deposits (Baker et al., 1987). Although radiocarbon dating yields
relatively reliable estimates of sediment age, it is time consuming andcomplicated; furthermore, in contrast to the precision of the age
estimates, because the geomorphology, hydraulic grade line (bed slope),

Geological features
Chausudake volcano Ⅰ
Chausudake volcano Ⅱ
OFUJIYAMA debris avalanche
deposits
NAKAGAWA debris avalanche
deposits
KUROISO debris avalanche deposits
SHIRAKAWA pyroclastic flow
deposit※Aira-Tn(AT)Depositional age
Since 16 thousand years ago
Since 150 thousand years ago
40～25 thousand years ago
170 thousand years ago
500 to 400 thousand years ago
1.4 to 1.0 million years ago
29 to 26 thousand years ago

Fig. 1 Geology of Yosasagawa Riv. basin
Meanwhile, no method currently exists to directly estimate historicalflood discharges on the basis of traces of erosive action. That said, it
may be possible to use regime theory, whose empirical equations havelong been used to determine stable channel cross-sectional geometry, toestimate the discharge forming a given channel from the channel crosssectional geometry(Knighton, 1998). There are, however, a number ofproblems with this approach, including the fact that the equations
constituting regime theory were developed empirically on the basis of
specific rivers and not on general conditions, and the fact that it is notpossible to determine whether present-day channel cross-sections wereformed by a single or multiple floods. To overcome these problems, in12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.

The current terrace
Fluff is the direction of inclination

confluence

Yosasagawa Riv.
river channel
valley wall

clear terrace of 70 years ago
unclear terrace of 70 years ago
confluence

5.2k
6.8k

7.2k

8.2k

Yosasagawa Riv.
Photo 1 Cutbank of the1998 flood

cutbank in 1998 flood
surface width of 1998 flood
Fig. 3 Erosion width of the 1998 flood、Step position by the 70-year-old aerial photograph interpretation

the present study we used the Fukuoka equation (a mechanicalrelationship equation) for dimensionless stable channel cross-sectional
geometry derived through dimensional analysis as the relationshipequation(Fukuoka, 2012). In addition, we focused on rivers running
through erosional valley-floor(valley plain) as sites where eroded
channel cross-sections formed by single historical floods might still
exist today.
During normal or small to medium flows, rivers that flow througheroded river valley floors meander along the valley bottom while
limited by the valley walls. However, an investigation of the 1998
flood of the Yosasagawa (a river that flows through Nasumachi,
Tochigi Prefecture) confirmed that during large-scale discharge events,
rivers cut the inner banks of bends along meandering river channels ina straight line and leave behind stream banks that lie in a straight line
along the axis of valley bottom (Photograph 1). We used the width of
this erosion as the channel width in the stable channel cross-sectional
geometry for the Fukuoka equation. In addition, we identified the highterraces oriented in the direction perpendicular to the long axis of thevalley floor as erosion traces formed through erosive action of a largescale historical flood. Using the Fukuoka equation, we attempted toestimate the extreme discharge based on this erosion width. Based on
this analysis, we estimated the extreme discharge of the Yosasagawa to
be approximately double that of the 1998 flood.

INVESTIGATION METHOD
Selection of target section of the riverIn this study, we focused on terraced landforms formed through theaction of rivers flowing along erosional valley-floor. In the context offluvial geomorphology, the main causes of terrace formation generally
include crustal movement (uplift and subsidence), changes in sea level(fall), and erosive action during large-scale flooding events. Terraces
formed via the first two mechanisms are known as fluvial terrace scarps.
Because we limited our investigation to terraces formed through theerosive action of large-scale flood events, the terraces have essentiallyvalley plains
terrass of a second stage
terrass of a first stage
cutbank in 1998 floodvalleywall

valleywall
Flood level of 1998

LegendKUROISO debris avalanche deposits
Deposits from five hundred thousand
years ago
Yellowish brown loam
Base is Aira-Tn(AT) of ca.26-29 ka、Surface is deposit increased activity
in the Chausudake volcano since 16
thousand years
Alluvium1
gravel (containing boulder)
OFUJIYAMA debris avalanche deposits
may be estimatedAlluvium2
unconsolidated sand or silt

Photo 2 terrace ofvalley plain in
Yosasagawa
Riv.

Fig. 2 Yosasagawa Riv. Geologic sketchy profile
been unaffected by crustal movement and sea level change in the recent
epoch. The largest flood event in recorded history occurred recently(1998), leaving a wide variety of flood-related records. For this reason,
we chose the Yosasagawa, which flows through Nasumachi, Tochigi
Prefecture, as our study target.
The Yosasagawa flows in a southeasterly direction from Mt. Nasu
(elevation 1,915 m), which is an active volcano. In the target section of
the river, the watershed catchment area is 127 km2, the main channel

12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.

length is 34 km, and the average grade is 1/32. The target section was a
5 to 9 km stretch of the lower reach of the Yosasagawa marked byprominent erosive features and a clear erosional valley-floor (Fig. 1).
In this study, although we assumed the terraces along the valley floor tobe erosion traces formed during a flooding event, we will first discuss
the possibility that they are faults or were formed by the falling sea
level. The Sekiya fault, Kenkatsura fault, and the Nasuyumoto-Hokuto
fault are known to exist near the upper reaches of the Yosasagawa inthe vicinity of the Mt. Nasu volcanic zone. However, no active faults
have been reported downstream of the study area along the Yosasagawa
or in the Nakagawa river basin, which is the main river. On the basis of
the existence and absence of active faults, in this study, we assumed theimpact of crustal movement to be insignificant.
As can be seen in the schematic diagram of valley floor terraces in Fig.
2, the terraces consist of brownish-yellow loam or unconsolidated sandto silt (N value smaller than 4 by a standard penetration test). The firstterrace is estimated to have been formed 26,000 years ago or later on
the basis of a trench excavation of historic site (Fig. 1); the second
terrace is believed to have been formed in the recent epoch (Holocene).
The sea level began falling 120,000 years ago during the last glacial
period but has been rising since 18,000 years ago. In other words, any
terraces formed due to sea level drop would have formed between
120,000 and 18,000 years ago. Given that the target section of theYosasagawa is located approximately at 80 km inland from thecoastline, we assumed that the influence of sea level drop was
insignificant.

Investigation method
The goal of this study was to explore the significance of lateral erosion
traces left during the 1998 flood (peak discharge of 1,740 m3/s) and thesignificance of the terrace landforms cut into the valley floor was
confirmed during our field survey. In our investigation, we (i) surveyed
the present day microtopography, interpreted historical aerial
photographs, and analyzed measurements taken at the time of thedisaster with the goal of understanding geomorphologicalcharacteristics of the study site, and (ii) examined existing geologicalmaps and geological survey results to understand the geologicalcharacteristics. In addition, we evaluated the composition of the surface
soil using a boring stick and analyzed the various hydrologic factors at
the time of peak flood discharge. We conducted our investigation while
comprehensively considering all of the above.

RESULTS

series in the same direction as the river indicate that the terraces existed
prior to farmland consolidation.
It is worth special mention that the lateral erosion caused by the 1998
flood widened the channel width three- to fivefold. Figure 3 indicatesthe locations of erosion traces on the left and right banks of thewidened channel. We defined the width of the cutbank to be the erosion
width.
Although erosion width varies considerably by location, by comparing
the terrace locations in the 1947 aerial photograph with present-day
terrace locations, we can ascertain that erosion has occurred on the
inner banks of the meandering river channel. We believe that these
represent erosion traces that were formed by water flows shortcutting
meandering segments of the river channel during a large-scale flood
event.
5.2k cross section
Geological section is discontinuous at the upper and lower terrace
Depositional age is different in the upper and lower
left bankfirst-terrassboring stick 1
Existing ground line
boring stick 2
Flood level of 1998
1.2m
cutbank
non-penetration (gravel)160m（erosion width of 1998）
221m（width in terrace of 70 years ago）

7.2k cross section
Loam is in the upper stage, not to lower.
⇒ This terrace is considered to have been formed by thelarge-scale flooding after 16,000 years
left bank
Right banksecond-terrass first-terrass
Existing
boring stick 2
ground line
boring stick 1
boring stick 3
Flood level of 1998
1m

cutbank

non-penetration (gravel)68m（erosion width of 1998）
129m（width in terrace of 70 years ago）

Geomorphological characteristics

Fig. 4 cross section of valley plain in Yosasagawa Riv.(10:1 format)

Photograph 2 shows terrace landforms observed along the valley floor.
Terrace height ranges from 0.3 to 1.2 m. At first glance, it appears thatthese landforms may have been formed during farmland consolidation.
In order to confirm that these terraces existed prior to farmland
consolidation, we interpreted aerial photos taken by the US armed
forces in 1947 (the oldest aerial photographs that still exist), which
predate substantial artificial improvement of the landscape,superimposed with the terrace locations. As can be seen in Fig. 3,
although there are minor discrepancies in a few locations, for the mostpart, the historical terrace locations match up with the present terrace
locations. This and that the terraces, although varying in height, lie inGeological characteristics
In order to clarify the terrace formation process, we confirmed thegeological continuity of the land interrupted by terraces by collecting
surface soil samples to a depth of 1 to 2 m (down to the impenetrable
pebble layer) using a boring stick.
We measured and surveyed terraces along four transverse transects at
representative locations where the valley floor widens (5.2 k, 6.8 k, 7.2
k, and 8.2 k) (Figs. 1 and 3). The results of this survey, shown in Fig. 4,
reveal the following:12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.

(1) We observed one or two terraces along each transect. Terraceheights ranged between 0.3 and 1.2 m. The terraces consisted of
two types: those that are 0.5 m or less in height and are difficult todiscern in the field, and distinct terraces that are on the order of 1 m
in height.
(2) For terraces that are on the order of 1 m in height, we were unable
to confirm the continuity of soil quality above and below the terrace(first terrace on the right bank at 5.2 k and second terrace on the leftbank at 7.2 k).
(3) The upper surface of the second terrace on the left bank at 7.2 k wasfound to consist of brownish-yellow loam. Although we did not
determine the age of this brownish-yellow loam, we are certain that
it was deposited before the most recent magma eruption of Mt.
Nasu, which occurred 600 years before present. Thus, we canconclude that the second terrace was formed through the erosive
action of a large-scale flood sometime after the deposition of theloam.

Fig. 5 shows a plot of the Fukuoka equation based on the relationshipbetween the dimensionless channel width and dimensionless discharge
for 5.2 k and 8.2 k. Using this curve, we then seek the dimensionlessdischarge associated with the dimensionless width of the first terrace. The
resulting discharge is calculated to be in the range Qp = 3,600 to 3,900
m3/s, which is approximately double the discharge of the 1998 flood.

DISCUSSION
Significance of the erosion width at points where lateralerosion traces were left by the 1998 flood
The valley walls of the Yosasagawa that flows along the erosional valleyfloor were formed 400,000 to 500,000 years ago by compacted
KUROISO debris avalanche deposits. Because the kuroisoiwa is highly
resistant to erosion, even if flow occurs toward the outer river bank,erosion width does not readily increase. In contrast, when flow occurs
toward the inner river bank, the loose Holocene sediment is readilyeroded, causing large-scale river bank recession and increased erosion
width.
In the 1998 flood, erosion width was large in some locations (5.2 k and8.2 k) and smaller in others (6.8 k and 7.2 k). We believe that erosion
width is determined by channel sinuosity, channel cross-sectional
geometry, water depth, and flow velocity.
If we use erosion width as the channel width for stable channel crosssectional geometry in the Fukuoka equation, as can be seen in Fig. 5,then data points for locations with large erosion widths (5.2 k and 8.2k) fall close to the mean curve of the Fukuoka equation; in contrast,
data points for locations with small erosion widths (6.8 k and 7.2 k)diverge substantially from the Fukuoka equation. These results suggestthat erosion width for cross-sections with large erosion widths represent
the appropriate channel width for determining stable channel crosssections.

Significance of terraces cut into the valley floor
In this study, we found that terraces on the order of 1 m in heightoccurred transverse to the direction of the valley floor. Given that thelocations of these terraces confirmed in aerial photographs taken by the
US armed forces in 1947, which predate any substantial artificial
improvement of the landscape, were slightly shifted toward the valley
wall compared to erosion traces of the 1998 flood (Fig. 3), we believethat terraces in locations such as 5.2 k and 8.2 k compatible with thepreviously mentioned Fukuoka equation that are also in locations where
there is only one terrace may represent erosion traces formed byextreme discharge as part of the process of valley floor formation.

Fig. 5 Correlation of dimensionless erosion width to dimensionless

CONCLUSIONS(1) This study demonstrates the possibility of utilizing historicgeological maps and aerial photographs and conducting detailed
microtopographic and geological surveys of the field site using
simple instruments such as boring stick to clarify the historicaldevelopment of river landforms and to estimate extreme discharge.
(2) We confirmed the location of terraces formed along an erosional
valley-floor believed to have been minimally impacted by crustal
movement and sea level change by using aerial photographs takenby the US armed forces in 1947, which predate any substantial
artificial improvement of the landscape, and surveyed the soil
quality and composition using a boring stick. High terraces on theorder of 1 m in height that also exhibit discontinuity in the soil
profile may have been formed by erosive action during a large-scale
flooding event.
(3) We believe that the development of flow on the inner bank-side ofmeandering channels varies depending on the interactions between
channel sinuosity, channel cross-sectional geometry, water depth,
and flow velocity and, further, that this flow impacts the magnitudeof erosion width.
(4) By thinking about the hydrologic characteristics of the Yosasagawa
that flows along the valley floor separately in terms of single cross-12th International Conference on Hydroscience & Engineering
Hydro-Science & Engineering for Environmental Resilience
November 6-10, 2016, Tainan, Taiwan.

sectional flow and compound cross-sectional flow, for each location,
we were able to estimate the extreme discharge that formed thevalley floor. Based on this analysis, we estimated the extreme
discharge to be approximately twice that of the 1998 flood.

REFERENCES
Baker, V.R., Pickup, G. (1987). Flood geomorphology of the Katherine
Gorge, Northern Territory, Australia. Geol. Soc. Amer. Bull. 98, 635646.
Knighton, D. (1998). Fluvial Forms & Processes, Arnold.
Fukuoka, S. (2012) Dimensionless width and depth and sedimenttransport rate in stable rivers, 3rd International Symposium on
Shallow Flows (ISSF).